This invention concerns radiation therapy, especially electronic x-ray brachytherapy, for treating tissues which may have diffuse proliferative disease.
In brachytherapy, a radiation source is generally placed within a surgically created or naturally occurring cavity in the body. In particular, this invention relates to delivery of x-ray therapy to tissue as might be found in the human breast, or to other tissue, preferably by activation of a miniature, electronic x-ray source. Such therapy often follows surgical treatment of cancer.
With conventional intracavitary brachytherapy, a prescribed dose is selected by the therapist to be administered to a volume of tissue (the target tissue) lying just outside the treatment cavity, into which a single radiation source will be placed.
Generally the prescribed dose will specify a uniform minimum dose to be delivered at a preferred depth outside the treatment cavity (the prescription depth). Also with conventional brachytherapy, since by the laws of physics radiation intensity falls off, essentially exponentially with increasing distance from the radiation source, it is generally desirable to create and maintain a space between the source of radiation and the first tissue surface to be treated (generally the cavity wall) in order to moderate the absorbed dose at the cavity surface while still delivering the prescribed dose at the prescription depth. This is usually accomplished by placing an applicator in the cavity which both fills and shapes the cavity into, most often, a solid figure of revolution (e.g., generally a sphere, cylinder or ellipse) and positions the radiation source within a source guide (i.e. within an applicator shaft) situated along a central axis of the cavity so formed and through which the source, usually part of a catheter assembly, may be traversed. If the applicator has a balloon to shape the cavity, it is preferably inflated using a fluid medium which has radiation attenuation properties similar to those of soft tissue. Water is such a medium. This choice of medium simplifies treatment planning.
One accepted standard in current breast cancer brachytherapy practice is a prescription depth of one centimeter beyond the treatment cavity surface, thus defining the target tissue, which is used for treatment planning. Assuming the tissue at the prescription depth receives the desired minimum dose, the tissue nearest the source (generally the cavity surface) should not receive more than 2.5 to 3 times the prescription dose (this is the allowable dose ratio). Current standards also require that the skin not receive a dose of more than about 1.5 times the prescription dose. With a one centimeter prescription depth, this usually requires the skin be at least 6-8 mm away from the surface of an applicator engaged against the tissue in the cavity. A distance of less than about 6-8 mm may result in doses higher than 1.5 times the prescription dose which are known often to result in undesirable patient cosmesis. Similar complications arise in proximity to bone and other tissues/organs as well. These proximity problems commonly arise and, since radiation overdose is to be avoided if at all possible, may well constitute a contra-indication for conventional isotropic breast brachytherapy. In brachytherapy application other than in the breast, prescription depths other than one centimeter may be preferred, but in general, the concerns and principles described above will still apply.
In order to assess the cavity shape and distances from cavity surfaces to skin surfaces or to other radiation sensitive structures, imaging of the cavity and apparatus is carried out as part of the planning process. Conventional x-ray imaging or CT scanning is often used for this purpose. If as is stated above, the volume of target tissue encompasses or abuts sensitive structures, the treatment planning process must be modified to reduce the dose exposure to such structures. If this can't be done as is often the case, brachytherapy as a treatment modality generally must be abandoned. With current planning algorithms, such accommodation is difficult.
It is apparent that methods and/or apparatus are needed which address the complexities described above, yet are compatible with current planning procedures, all of which would make brachytherapy an option for a greater proportion of the patient population, and more effective when applied.